The ultimate objective of the proposed investigation is to elucidate the mechanism by which the "non-cytopathogenic" lactate dehydrogenase-elevating virus (LDV) only replicates in macrophages and similar types of cells and how virus replication operates within the framework of the regulatory processes of the host cell. The underlying question is how LDV affects various macrophage functions as exemplified by its inhibitor of blood enzyme clearance and its possible effect on the immune systems of the host. Related is the question of why only a relatively small proportion of macrophages in primary cultures can be productively infected with LDV. This has been recently discovered by the use of an autoradiographic method developed by us. We will approach all these questions by monitoring various macrophage functions, Fc and C3 receptors, and LDV replication in individual cells of macrophage populations at various times after placing the cells in culture. Macrophages will be propagated in mouse L-cell conditioned medium, which stimulates the replication of these cells in vitro. Antibodies to LDV have been isolated from chronically infected mice and conjugated with fluorescein isothiocyante. It will be used in conjunction with above methods to assess LDV replication in individual cells of infected cultures or mouse tissues. We will also use anti-LDV antibodies to develop an in vitro procedure for its titration. Other studies concern the question of whether the transient high level production of LDV in macrophages and the following chronic persistent infection are related to interferon production. The question of whether LDV enhances tumor induction by RNA tumor viruses will be investigated by studying the replication of LDV and mouse RNA tumor viruses in long-term macrophage or lung-spleen cultures. Studies on the physical-chemical characteristics of LDV and the method of replication will be continued. A search will be made in other species, including man, for viruses of a type similar to LDV. Human blood samples will be screened for elevated enzyme plasma levels and suspected samples will be analyzed for virus by inoculation into human macrophage cultures.